An antenna system corresponding to multiple types of communication systems and including multiple antennas is known. When the multiple antennas respectively corresponding to the multiple communication systems are disposed adjacent to each other, radio wave interference between the communication systems causes a problem.
In an antenna apparatus disclosed in JP-A-2010-004318 (corresponding to US 2011/0122039 A1), a radiation element of one antenna and a radiation element of the other antenna are disposed in a twisted relationship so that isolation of two antennas used for two communication systems is secured.
In other words, in the antenna apparatus disclosed in JP-A-2010-004318, the one antenna is disposed in a plane orthogonal to a substrate while being inclined 45 degrees with respect to the substrate. The other antenna is disposed in a plane orthogonal to the substrate and apart from the plane including the one antenna while being inclined 45 degrees in a direction opposite from the one antenna.
When one of communication systems is a communication system using a polarization diversity (hereafter, referred to as a polarization diversity communication system), it is required not only to enhance isolation between multiple communication systems but also to secure a communication performance of the polarization diversity communication system.